1. Field of the Invention
The present invention relates to a vehicle steering control system for making vehicle response characteristics equal to desired response characteristics by varying a manipulated variable of a steering system such as a front wheel steer angle and/or a rear wheel steer angle, and more specifically to such a vehicle steering control system providing an optimum control performance by preventing abrupt change in vehicle behavior during a turn.
2. Description of Prior Art
In a conventional vehicle equipped with a mechanical steering linkage, the front wheels are steered in accordance with an angular displacement of the steering wheel. Therefore, the vehicle steering response characteristics are fixed by vehicle parameters which represent some characteristic properties or dimensions inherent in the vehicle. That is, the vehicle steering response characteristics are fixed by the make of the vehicle. Therefore, the possibility of improvement in steering response characteristics is very limited.
3. Description of Related Art
In the above-mentioned copending application Ser. Nos. 755,352, 773,650 and 773,622, for example, there are proposed vehicle steering control systems of a so-called model solving type in which a controller first solves a desired model simulating a motion of a desired vehicle by means of equations of vehicle motion and values of vehicle parameters to determine a desired value of a vehicle motion variable corresponding to a steering wheel angle and a vehicle speed, secondly solves an actual vehicle model representing an actual controlled vehicle to determine at least one of front and rear wheel steer angles requires to make the actual value of the motion variable of the controlled vehicle equal to the desired value, and controls the actual steer angle in accordance with the desired steer angle value. The control systems of this type can control the vehicle response characteristic freely.
In order to perform the vehicle steering control more accurately, the control system of the above-mentioned copending application No. 837,170 is arranged to sense the vehicle motion variable which is relatively easy to sense, with a sensor, and feed back the sensed motion variable value. In this system, the actual vehicle model is modified by a so-called parameter identification process so as to make the vehicle motion variable of the actual model equal to the sensed motion variable value. However, the arrangement performing the parameter identification in a real time manner tends to deteriorate the control and stability of the vehicle by changing the characteristics of the actual model abruptly during a turning motion of the vehicle.